Treatment of biological tissues using subnanosecond electric pulses

ABSTRACT

A system for treatment of biological tissues is provided. The system can deliver electric pulses to a targeted region within a biological tissue. The system includes an antenna assembly and a lens. The antenna assembly is configured to generate and direct electromagnetic radiation. The lens is configured to be positioned between a surface of the biological tissue and the antenna assembly. The lens can have a plurality of lossy portions. The lens can be configured to be adjustable to create a patient-specific desired electric field distribution by selective positioning of the plurality of lossy portions within the lens.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.15/151,149, filed on May 10, 2016, which is a continuation of U.S.application Ser. No. 14/170,720, filed on Feb. 3, 2014, which claimspriority to Provisional Application Ser. No. 61/759,586, filed on Feb.1, 2013. The entire contents of all of the above-identified applicationsare incorporated herein by reference as if fully set forth herein.

GOVERNMENT LICENSE RIGHTS

This invention was made with government support under contract no.FA95500810191 awarded by the U.S. Air Force Office of ScientificResearch (AFOSR). The government has certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates to neurostimulation, and more specificallyto apparatus and methods for treatment of biological tissues usingsubnanosecond electric pulses.

BACKGROUND

Neurostimulation has been increasingly used as a therapy in areas whereconventional pharmacological approaches become ineffective, such as intreating refractory pain, Parkinson disease, dystonia, and obsessivecompulsive disorder. The stimulated structures may vary greatly. Forexample, in treating neuropathic pain, the neurostimulation techniquesinclude transcutaneous electrical nerve stimulation (TENS), peripheralnerve stimulation (PNS), nerve root stimulation (NRS), spinal cordstimulation (SCS), deep brain stimulation (DBS), epidural motor cortexstimulation (MCS), and repetitive transcranial magnetic stimulation(rTMS)¹. A specific example is using MCS to treat trigeminal neuropathicfacial pain, a syndrome of severe, constant pain due to pathologicalchange or injury to the trigeminal system². The target is locatedanterior to the central sulcus and posterior to the splitting of theinferior frontal sulcus.

Most neurostimulation methods are invasive and rely on electrodes thatare implanted into the stimulated structure through intraoperativesurgery. A pulse generator and a battery must be implanted in the body,often causing complications such as infection, lead migration, hardwaremalfunction, battery failure or unwanted stimulations. It was reportedthat, overall, 43% of patients experience one or more complications³.Significant research has been conducted on the use of rTMS, anon-invasive approach for brain stimulation. In rTMS, electric fieldsare induced by a fast changing, magnetic flux to cause the stimulation.The electric field penetration depth is limited to a 2 cm⁴, and theexcitable volume is not less than ten cubic centimeters⁵. Besideselectrical stimulation, ultrasound energy was used for neurostimulationstudies⁶, aiming to improve the focus of the stimulation and to accessdeeper brain zones, but the significant difference in the sonicproperties between the brain tissue and the bone, as well as theirregular skull shape prevent the focusing of the power⁷. Recently,optogenetic approaches revolutionized the neurostimulation using light,but the expression of photon-sensitive ion channels is required and theimplantation of a light source is needed⁸.

SUMMARY

Embodiments of the invention concern systems and methods for thetreatment of biological tissues. In a first embodiment of the invention,a system for treatment of biological tissues is provided. The system candeliver electric pulses to a targeted region within a biological tissue.The system includes an antenna assembly and a lens. The antenna assemblyis configured to generate and direct electromagnetic radiation. The lensis configured to be positioned between a surface of the biologicaltissue and the antenna assembly. The lens can have a plurality of lossyportions. The lens can be configured to be adjustable to create apatient-specific desired electric field distribution by selectivepositioning of the plurality of lossy portions within the lens.

In some examples, the lens can comprise a plurality of layers. Eachlayer can have a different value of conductivity. The plurality of lossyportions can be positioned within a single layer of the plurality oflayers.

In some examples, the plurality of lossy portions can be positionedwithin multiple layers of the plurality of layers.

In some examples, the plurality of lossy portions can be positioned atvarious incident angles of the lens.

In some examples, each of the plurality of lossy portions can beconfigured to attenuate a portion of the electromagnetic radiationtraveling in at least one direction.

In some examples, the targeted region can be a deep tissue region of thebiological tissue. The system can be configured to increase atransmission of the electromagnetic radiation to the deep tissue regionof the biological tissue. A focal spot in the deep tissue region can begenerated by attenuating the electric fields on or near a z-axis.

In some examples, the deep tissue region can comprise a region of 6 to 8centimeters (cm) below a surface of the biological tissue.

In some examples, the system configuration can simultaneously targetboth deeper and shallower regions of the biological tissue.

In some examples, the electromagnetic radiation from the antennaassembly can be configured to enter the targeted tissue at wide angles.The wide angles can be equal or greater than 45 degrees.

In some examples, the antenna assembly can comprise a wide-band antennaconfigured to provide wide and narrow angles.

In some examples, the system ca n further comprise a pulse generatorcoupled to the antenna assembly and configured for generating a seriesof pulses. The pulse generator can be configured for generating a pulsehaving a pulse duration of less than or equal to 1 ns. Alternatively, orin addition, the pulse generator can be configured for generating apulse having a magnitude between about 1 kV and 1 MV and a repetitionrate between about 1 Hz and 1 MHz.

In some examples, at least one of a rise time or a fall time of theelectric pulses can be from 1 ps to 1 ns.

In some examples, the lens can comprise a plurality of layers and atleast one of a radius, thickness, or dielectric constant of each layerof the plurality of layers varies.

In some examples, the antenna assembly can comprise an antennaconfigured for receiving the electric pulses and generating theelectromagnetic radiation and a reflector configured for directing theelectromagnetic radiation to the lens.

In some examples, the lens can be shaped and can be configured forpositioning adjacent to the biological tissue. A dielectric constant ofa surface of the lens that is adjacent to the biological tissue cansubstantially match a dielectric constant of the biological tissue.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B show front and side views of an arrangement of anantenna assembly with respect to a head of a patient without a lens inaccordance with the various embodiments;

FIGS. 2A and 2B show front and side views of an arrangement of anantenna assembly with respect to a head of a patient with a lens inaccordance with the various embodiments;

FIG. 3 shows a cutaway view of an exemplary configuration for lens inFIGS. 2A and 2B.

FIG. 4 schematically shows an arrangement of an antenna assembly andhemispherical shaped tissues to be treated in accordance with thevarious embodiments;

FIG. 5 is a more detailed view of the antenna assembly of FIG. 4;

FIG. 6 is a schematic of a spatial arrangement of components inaccordance with an exemplary embodiment of the invention;

FIG. 7A shows the dielectric lens consisting of five layers is used inconjunction with the reflector antenna of FIG. 6 and hemisphericalshaped tissues;

FIG. 7B shows the dielectric lens consisting of five layers is used inconjunction with the reflector antenna of FIG. 6 and a patient head;

FIG. 8A shows a plot of a Gaussian pulse used in the variousembodiments;

FIG. 8B shows a plot of the electric field pulse at a focal point as afunction of time;

FIG. 9A shows a plot of the electric field distribution on the z-axisfor different tissue conductivities;

FIG. 9B shows a plot of the electric field distribution for highertissue dielectric permittivity;

FIG. 10 shows a plot of the field distribution in the radial directionat different angles θ;

FIG. 11 shows a plot of the field distribution of impulse for tissueconductivity of 0.5 S/m when the lens has no lossy layers;

FIG. 12A shows a plot of the electric field distribution along paths atdifferent azimuth angles when a partially lossy lens is used in additionto the reflector antenna and where the conductivities of the last twolayers of the lens are 1 S/m;

FIG. 12B shows a plot of the electric field distribution along radialpaths when the conductivities of the last two layers are 1S/m and 2 S/m;

FIG. 13A schematically shows the case where a partially lossy lens isused and the distribution of electric field in the target shows a focalpoint formed in the deep zone of the target and two hot spots on thetarget surface;

FIG. 13B schematically shows the case where a non-lossy lens is used andthe electric field distribution in the hemispherical tissue can beschematically depicted in a pie shape;

FIG. 14 is a plot of the pulse waveform at the 6 cm depth on the z-axisin the brain exposed to a reflector antenna;

FIG. 15 shows the isoline plots of electric field distribution in thehuman brain without a dielectric lens at various times with a clamp torange of 0.1 to 0.3 along the X axis (side view), Y axis (back view) andZ axis (top view), where the top-view slice is taken along the plane ofthe lens at 6, 7, 8 and 9 cm deep in a human head;

FIG. 16 is a plot of the electric field distribution in the brainexposed to the reflector antenna without the dielectric lens;

FIG. 17 shows the isoline plots of electric field distribution of thehuman brain with a dielectric lens according to the various embodimentsat various times along the along the X axis (side view), Y axis (backview) and Z axis (top view), where the top-view slice was taken alongthe plane at 8 cm deep in a human head;

FIG. 18 shows a plot of the electric field distribution in the brainwith the non-lossy lens; and

FIG. 19 shows a plot of the electric field distribution in the brainwith the lossy lens.

DETAILED DESCRIPTION

The present invention is described with reference to the attachedfigures, wherein like reference numerals are used throughout the figuresto designate similar or equivalent elements. The figures are not drawnto scale and they are provided merely to illustrate the instantinvention. Several aspects of the invention are described below withreference to example applications for illustration. It should beunderstood that numerous specific details, relationships, and methodsare set forth to provide a full understanding of the invention. Onehaving ordinary skill in the relevant art, however, will readilyrecognize that the invention can be practiced without one or more of thespecific details or with other methods. In other instances, well-knownstructures or operations are not shown in detail to avoid obscuring theinvention. The present invention is not limited by the illustratedordering of acts or events, as some acts may occur in different ordersand/or concurrently with other acts or events. Furthermore, not allillustrated acts or events are required to implement a methodology inaccordance with the present invention.

In view of the limitations of conventional neurostimulation systems andmethods, the present invention provides a new neurostimulation methodusing intense, high power, subnanosecond electric pulses (psEP) asstimuli. In some embodiments, such electric pulses can be delivered byultrawideband antennas in the form of electromagnetic waves, which havethe potential to penetrate deeper with a higher resolution. For example,a 200 ps subnanosecond electric pulse contains wideband frequencies upto 5 GHz, according to the reciprocal relation between pulse durationand frequency. Previous numerical and experimental studies suggest it ispossible to focus high frequency radiation using an antenna array. In abrain hyperthermia study, the array of four dipole antennas spaced 2.0cm apart was capable of heating a volume of 5.9 cm×2.8 cm×2.8 cm⁹. Dunnet al.¹⁰ showed that careful selection of the source electric fielddistribution around the entire surface of the head can generate a wellresolved focus. Gouzouasis et al.¹¹, in a deep brain monitoring system,showed electric fields at 1.1 GHz that are radiated from a disconeantenna together with an ellipsoidal reflector can be focused on thecenter of the head. A beamforming approach was shown to focus 1 GHzmicrowave radiation through constructive interference to treat braincancer by Burfeint et al.¹² in a modeling study.

However, a significant issue with such conventional treatment methods iswhether or not there is sufficient transmission, to the tissues ofinterest inside a patient's head, of pulse radiation resulting fromsubnanosecond electric pulses (psEPs). This is described with respect toFIGS. 1A and 1B. FIGS. 1A and 1B show front and side views of anarrangement of an antenna assembly 102 with respect to a head of apatient 104. As shown in FIGS. 1A and 1B, the antenna assembly 102consists of an ellipsoidal or prolate spheroid reflector 106 and adiscone antenna 108. In such a configuration, although the pulseradiation would be delivered to the patient 104, the psEPs may notpenetrate the patient in an amount sufficient for therapeutic purposesdue to reflection losses and incident angles. For example, if the pulsesare sent from the air to a tissue which has a dielectric permittivity of80, about 80% of the electric field is reflected due to the largecontrast of dielectric property.

Thus, another aspect of the various embodiments is to provide aconfiguration that increases the transmission of pulse radiation frompsEPs to the regions of interest in the brain. First, to increase thetransmission of radiation to deep tissues, the pulse radiation from thereflector antenna is configured so as to enter the tissue at wideangles. Secondly, the reflection loss due to the abrupt change in tissuepermittivity at the interface of air and tissue is minimized.

Thus, the various embodiments utilize a combination of a widebandantenna, capable of wide and narrow angles, and an inhomogeneous,partially lossy dielectric lens. An exemplary arrangement for such aconfiguration is illustrated in FIGS. 2A and 2B.

FIGS. 2A and 2B show front and side views of an arrangement of anantenna assembly 202 with respect to a head of a patient 204. As shownin FIGS. 2A and 2B, the antenna assembly 202 consists of an ellipsoidalor prolate spheroid reflector 206 and a discone antenna 208, similar tothe configuration of FIGS. 1A and 1B. However, the arrangement in FIGS.2A and 2B further includes an inhomogeneous, partially lossy dielectriclens, specifically a multilayer lens 210 to match the impedance from airto tissue. This is illustrated with respect to FIG. 3.

FIG. 3 shows a cutaway view of an exemplary configuration for lens 210in FIGS. 2A and 2B. In some embodiments, as illustrated in FIG. 3, thelens 210 can be configured as a hollow, hemispherical or substantiallyhemispherical with a hemispherical or substantially hemispherical cavity310. As used herein, the term “substantial” or “substantially” refer tobeing within 20% of the stated value or property.

The cavity 310 can be used to locate the lens 210 around tissues. Thus,the cavity 310 (and the lens 210) can be sized in accordance with thetissues to be treated. For example, the cavity can be configured to beplaced on and over a portion of a patient's head, as shown in FIGS. 2Aand 2B. In another example, the cavity can be configured to be placedover a female breast. In such a configuration, the cavity 310 can have aradius corresponding to the size of the breast of the patient. However,the various embodiments are not limited in this regard and the lens andcavity size can vary in the various embodiments.

As further shown in FIG. 3, the lens 210 can be defined using fivelayers, layers 302, 304, 306, 308, and 309, of different dielectricmaterials, where each layer has a hollow, hemispherical or substantiallyhemispherical shape. The dielectric materials and their order in lens210 can be selected so that the dielectric constant from varies insubstantially monotonic profile. In some embodiments, this can be anexponential profile. The exponential profile can range from a dielectricconstant of air or other ambient to a dielectric constant of the tissuesin cavity, ϵ_(rmax). Due to the increase of the dielectric constantprovided by such a configuration, the focal spot size can be reduced bya factor of ϵ_(rmax) ^(−1/2) at the innermost layer, and the electricfield can be enhanced by a factor of ϵ_(rmax) ^(1/4). In the variousembodiments, the configuration of lens 210 can be selected in a mannersimilar to that described by Altunc et al. in “Focal waveforms forvarious source waveforms driving a prolate- spheroidal impulse radiatingantenna (IRA),” Radio Sci. 43: RS4S13 (2008). However, in the variousembodiments, the lens 210 is configured with two major differences.First, Altunc et al. describe a lens with five (5) layers and no cavity.That is a solid hemispheric ally shaped lens. In contrast, the lens 210is configured such that the patient's tissues define the inner most theinnermost layer of the lens, as described above. Second, the innermostlayers 308, 309 of the lens 210 are configured to be lossy for anglesbelow 45°. This means that for azimuth angles between −45° and +45°, thelens 210 has a finite conductivity (2 S/m).

As noted above, the radius, thickness, and dielectric constant of eachof layers 302-309 can vary. In one particular, configuration, thethicknesses of layers 302, 304, 306, 308, and 309 can be 3 cm, 2.3 cm,1.9 cm, 1.6 cm, and 0.2 cm, respectively. The dielectric constants ofthese layers are 1.6, 2.4, 3.7, 5.8, and 9.0, respectively. Theinnermost layer, layer 309 is selected to have a dielectric constantapproximately equal to the dielectric constant of the biological tissuein the cavity. In some embodiments, the radius of the cavity can varyfrom a few centimeters (1-3 cm) to approximately 10 cm (8-12 cm), wherethe radius is selected in accordance with the tissues and structures inthe cavity. However, other radii outside this range can be provided. Asnoted above. The materials for the lens can be the composite ofnanometer-sized metal oxides, polymers, or ceramics, which have thedielectric permittivity close to the tissue value. Some liquids, such ascorn syrup, can be used in a gel form and work as the innermost layer ofthe lens.

In an alternate embodiment, the lens can include one or more lossyelements along radial directions. These lossy elements can be selectedso as to attenuate the electromagnetic radiation in at least onepre-defined radial path to control the angle of the power incidence.

As shown in the configuration of FIGS. 2A and 2B, the antenna assembly202 consists of a reflector 206 and a discone antenna 208. However, thevarious embodiments are not limited to solely this configuration. Asnoted above, the primary requirement with respect to the pulse radiationis that such radiation enters the patient's tissues at wide angles. Thisis illustrated with respect to FIGS. 4 and 5. FIG. 4 schematically showsan arrangement of an antenna assembly 402 and hemispherical shapedtissues 404 to be treated. FIG. 5 is a more detailed view of the antennaassembly.

In FIGS. 4 and 5, the antenna assembly 402 consists of an ellipsoidalreflector 406 and a discone antenna 408. The discone antenna 408consists of vanes 408 a defining a cone and a disc 408 b to which asignal from a pulse generator (not shown) is fed. The disc 408 b ispositioned at a first focal point (F1) of the reflector 402 and thetarget tissues are positioned at a second focal point (F2) of thereflector 402, such that the hemispherical shaped tissues, i.e., thepatient 404, a lens 405, the disc 408 b, the vanes 408 a, and thereflector 402 are placed so that they are substantially symmetric withrespect to an axis 410 defined by the focal points F1 and F2.

In order to provide wide angle for the patient's tissues, the reflector406 is configured so that pulse radiation from focal point F1 impingingon edge X of reflector 406 is reflected toward focal point F2 so that anangle θ with respect to axis 410 is greater than 45 degrees. Suchconfiguration can be utilized for directing and focusing pulse radiationat focal points deep within the hemispherical shaped tissues of thepatient 404 to be treated. However, the present disclosure contemplatesthat in some configurations, it shallower tissues may need to betreated. In such configurations, the reflector 406 can be configured toprovide an angle θ less than 45 degrees.

In operation, the system described above is utilized as follows. First,the patient 404 and the dielectric lens 405 are positioned, with respectto the antenna assembly 402, in the manner described above. Inparticular, as described above, the patient is positioned such that thetarget area T within the patient 404 to be treated (e.g., area withinthe head of patient 404) is positioned at the second focal point of thereflector of the antenna assembly. Once positioned, a pulse generator414 can be utilized to generate the psEPs.

In the various embodiments, the actual width of the psEP can vary. Forexample, the pulse widths can be between about 1 ps and 1 ms. However,of greater importance in the various embodiments, with respect toneurostimulation, is the transient of such pulses, i.e., the width ofthe rise time and the fall time of such pulses, which can be 1 ps to 1ns. That is, the useful portion of the electromagnetic radiation beinggenerated is the high frequency component associated with thetransitions in the pulse sequence. Such pulse transients are capable ofbeing radiated by antennas of 1 meter or less. In particular, thetransient of such pulses are important since the length of the transientcan affect whether or not neurons shut down. Specifically, the powerdelivered to biological tissues for treatment will vary as the transientvaries. Thus insufficient stimulation may be provided. For example, ifthe transients are too long, the power delivered is spread out andcannot be accurately focused in a single area. In some embodiments, thepsEPs can be configured to have a transient be between about 1 ps to 1ns, such as 100-500 ps, or 200 ps. These pulses can have voltagesbetween about 1 kV and 1 MV. These provide electric fields magnitudesbetween about 1 V/cm to 1 MV/cm, such as between about 1 kV/cm to 500kV/cm, or 100 kV/cm.

In some embodiments, the pulses can be delivered as series ofconsecutive pulses. This allows for the accumulation of depolarizationin neurons before action potentials fire. In these embodiments, thepulses can have a repetition rate between about 1 Hz to 1 MHz. In someconfigurations, where additional neurostimulation is required, therepetition rate is increased while maintaining the transient of thepulses constant.

It should be noted that the configuration of the antenna assemblyillustrated in the preceding figures is provided solely for illustrativepurposes and not by way of limitation. Rather, the present disclosurecontemplates that the pulse radiation to be delivered to hemisphericalshaped tissues via a hemispherical lens can be generated and deliveredin a variety of ways. That is, the antenna assembly can include any typeof antenna for generating pulse radiation based on psEPs and any type ofreflector, lens, or directing device for directing the pulse radiationat the desired angles. Other suitable antenna configurations include,such as a resistive-loaded dielectric rod or dipole antenna, an antennaarray which includes a number of antennas, or a resistance terminatedtransmission line antenna.

Other exemplary antenna assemblies suitable for the various embodimentsare described in U.S. Pat. No. 8,000,813, the contents of which areherein incorporated by reference in their entirety.

Although the various embodiments have been described primarily withrespect to neurostimulation, the present invention is not limited inthis regard. In some embodiments, the psEPs and resulting pulseradiation can be configured for other therapeutic purposes. For example,the systems and methods described herein can be adapted for purposes ofinducing cell death in tissues of interest. Alternatively, the methodsdescribed herein can be adapted for purposes of increasing cell membranepermeability to allow for the delivery of therapeutic agents insidecells. These agents can include chemical or pharmacological agents andbiological agents for treatment of cells or inducing cell death.

Further, although the various embodiments have been described generallywith respect to a lens positioned on top of a hemispherically shapedbody part, such as female breast or a head, the various embodiments arenot limited in this regard. Rather, the systems and methods describedherein can be utilized for placing the lens on any body part to treatany type of biological tissue. In the various embodiments, this can beaccomplished in a variety of ways. In some embodiments, the inner layerof the lens can be molded to fit the non-hemispherically shaped bodypart. In other embodiments, the non-hemispherically shaped body part canbe placed in the cavity with a gel, liquid, or solid mold with a similardielectric constant to provide a hemispherically shaped object in thecavity. Similarly, other non-hemispherically shaped biological tissuescan be used in a similar fashion.

EXAMPLES

The following examples are provided solely for illustrating variousaspects of the present invention and should not be considered limitingin any way.

The following study is based on the inclusion of a hemisphericallyshaped tissue as the target in the antenna configuration. This study isdirected to the varying of the electrical properties (permittivity andconductivity) of the tissue and thereafter examining the spatialdistribution of the electric field intensity in the target, particularlyin the deep region, which is defined as 6-8 cm below the surface.Further, the temporal development and the spatial distribution of theelectric field intensity in a human brain is also examined, with the aimof determining the electric pulse parameters required for deep brainstimulation³². In particular, the study was directed to answering thefollowing questions:

1. How deep can the subnanosecond pulses penetrate and still achieve areasonable confinement of the electric field distribution?

2. Can one use one antenna to focus pulsed radiation into tissue?

3. What spatial resolution can such antenna provide?

To answer these questions, 3-D electromagnetic simulation software, CSTMICROWAVE STUDIO, available from CST of America, Inc. of Framingham,Mass., has been used. The transient solver based on Finite IntegrationTechnique (FIT) is used for the time domain simulation. The human modelis a HUGO human body model, also available from CST of America, Inc. ofFramingham, Mass., which has a resolution of 1 mm×1 mm×1 mm.

For this study, the basic configuration (reflector antenna and target)used in this modeling study are substantially similar to that shown anddescribed above with respect to FIGS. 2A, 2B, 4, and 5. The antenna hasa prolate spheroid reflector fed by a TEM conical transmission linestarting from the first focus as a wave launcher or disc. The reflectoris one half of a prolate spheroid with the minor axis on the apertureplane. The geometry of the antenna was taken from an existing antenna¹³,constructed by Fan Research of Albuquerque, N.M. The features of theantenna include: linearly polarized radiation, broadband radiation, andhigh power capability. The radiation at the second focus is verticallypolarized, which is the mirror image of the electric field at thestarting point of the transmission line fed by a differential input.Dimensions utilized for this study are shown in FIG. 6.

A lens was used in conjunction with the impulse radiating antenna,aiming to improve the coupling of the EM energy to tissue, as discussedabove with respect to FIG. 3. The lens is made of multiple layers withdielectric permittivity varying according to an exponential profile inorder to allow for a maximum transmission²⁹. The innermost layer has thesame dielectric constant as the targeted tissue.

In the simulation, a tissue target was used that has a hemisphericalshape, as shown in FIG. 7A. FIG. 7A shows dielectric lens consisting offive layers is used in conjunction with the reflector antenna. The lastlayer of the lens is adjacent the biological tissue. The biologicaltissue has radius of 6.0 cm. The dielectric constants of these fivelayers from the outermost to the innermost are 1.6, 2.4, 3.7, 5.8, and9.0, respectively. The biological tissue (innermost layer) has the samedielectric constant as the 5th layer.

Such shape was chosen because the spherically incoming waves from thereflector antenna have the same phase along the circumference of thetissue, which results in a maximum field at the second focus for thesame optical path. The relative permittivity of muscle and fat tissue ison the order of 45 (for muscle) and 5 (for fat). Their conductivity isin the range of 0.1-2 S/m. The values could vary for real tissues¹⁷. Thedielectric properties are dependent on the frequency, which can bedescribed by the second order Debye model¹⁸. This type of model is notavailable in the software, so not included are the effects ofdispersion. The dispersion of the dielectrics in general broadens thepulse waveforms and lowers the pulse intensity, the results of thesimulation without dispersion loss therefore serves as the best-scenarioestimate of the pulse delivery in terms of pulse intensity and focalspot size.

In addition to the hemispherical target, the HUGO human body model wasused to determine the temporal and spatial distribution of the pulsedelectric fields in a human brain, as shown in FIG. 7B. In the human headmodel used in this study, the dielectric constants and conductivities ofthe tissues considered in this study are: bone (ϵ_(r)=11.78, σ=0.28),gray matter (ϵ_(r)=50, σ=1.39), fat tissue (ϵ_(r)=5.35, σ=0.08), nervusopticus (ϵ_(r)=30.87, σ=0.84), white matter (ϵ_(r)=37, σ=0.91) andskeletal muscle (ϵ_(r)=55.33, σ=1.44). The heterogeneity of the tissuesmay cause multiple reflections and reduce the transmission of theelectromagnetic wave energy. In this study, focus is on the attenuationof the pulse in the brain, the size of the focal volume and the requiredinput pulse parameters to create the electric fields that are at thelevel of inducing any biological effects. It should be noted that theaccuracy of the CST software is satisfactory. As reported previously¹⁵,using the same software to simulate the impulse radiating antenna gaveresults very close to the measurement results. One can therefore assumethat the simulation yield is sufficiently accurate in predicting theeffect of wideband pulses on a human brain.

The electromagnetic waves emitted from the prolate spheroid reflectorare spherical waves, converging at the second geometric focus of thereflector. The phases of the incoming spherical waves on thecircumference of the target are identical. Because the electric fieldsof the converging spherical waves are parallel to the surface of thehemispherical tissue, the transmittance into the tissue is optimum¹⁹. Inthe simulation, the relative permittivity of the tissue has been variedfrom a value of 9 to a value of 70, and the conductivity was also variedfrom 0 to 0.5 S/m. Zero conductivity corresponds to a lossless tissue.The tissue conductivity near 0.5 S/m is close to that of breasttissue¹⁷.

Input Gaussian pulses (with a 200 ps transient) were fed into theantenna. A schematic of such a pulse is shown in FIG. 8A. For a relativepermittivity of ϵ_(r)=9 and a conductivity of the target of σ=0, theelectric field was sampled along the z-axis from the tissue surface tothe geometric focus of the reflector, which is 6 cm deep. The electricfield pulse at the focal point is shown in FIG. 8B. This electricalfield pulse consists of prepulse and an impulse. The prepulse reachesthe second focal point at an approximate time, t=27.16/30+6*(√9)/30=1.505 ns. The electric field pulse reflected from the prolate spheroidreflector surface converges at the second focal point F₂. Thisconstitutes the impulse part of the wave at the focal point and occursat approximately t=54/30+6*(√9)/30=2.4 ns. The shape of the impulse isclose to the time derivative of the Gaussian wave form²⁰. The electricfield component that contributes mostly to the electric field at thefocal point is the y-component. The amplitudes in x and z direction arenegligible. In FIG. 9A, the electric field distribution on the z-axisfor different tissue conductivities (s) is shown. The tissue dielectricpermittivity (e) is fixed as 9. The desired focus, also the geometricfocus, is at z=6 cm. While the electric field can be focused for tissueconductivities less than 0.3, it decreases monotonically along z-axisfor higher tissue loss (s=0.5). In FIG. 9B, the electric fielddistribution for higher tissue dielectric permittivity is shown. Shownin the figures are the absolute values of the electric field for aninput voltage of 1 V.

The electric field intensities along the z-axis for ϵ_(r)=9 and forvarious values ofσ are plotted in 1 cm steps from the surface to thetarget in FIG. 9A. It can be seen clearly that the field has its maximumat the second focus. However, this focusing effect diminishes as theconductivity increases from 0.3 S/m to 0.5 S/m: the maximum field islocated on the surface and it decreases monotonically on the z-axis. Thedecreasing trend of the z-axis electric field however is reversed whenthe tissue permittivity was changed from 9 to 70, as shown in FIG. 9B.With the tissue dielectric permittivity of 70, the field is focused atthe geometric focus even for a conductivity of 1 S/m. Clearly, thefocusing effect is more pronounced in tissue with higher permittivity.

The field distribution in the radial direction at different angles θ isshown in FIG. 10. FIG. 10 shows the electric field distribution in theradial direction at different azimuthal angles (θ). At the surface ofthe tissue hemisphere, the highest electric field is near the z-axis,but it decreases to 60% at the geometric focus. In other radial pathswith angles greater than 30°, the field at the surface is eitherapproximately equal or smaller than the field at the geometric focus.The tissue has dielectric permittivity (ϵ=9) and conductivity (σ=0.5).

In this case, the tissue dielectric constant is 9 and the conductivityis 0.5 S/m. The electric field amplitudes on the z-axis as well as thoseof the electromagnetic wave with a small azimuthal angle) (<30°) in theradial direction show a decreasing trend as they propagate into thetissue. On the other hand, the waves traveling in the radial directionwith larger angles decrease over the first 1-2 cm, but increaseeventually at the destination (6 cm in depth). The differences in lossesalong different radial paths suggest the possibility to generate a focalspot in the deeper tissue by attenuating the electric fields on or nearthe z-axis, and letting only the electric fields along paths of widerangles continue to propagate. As will be shown below, this can beachieved by manipulating the properties of the lens.

In order to maximize the electrical energy density in a focal spot inthe deep tissue, two conditions need to be satisfied. First, as brieflydiscussed above, the amplitude of the electromagnetic wave at smallangles needs to be attenuated, and secondly, the reflection loss due tothe abrupt change in tissue permittivity at the interface of air andtissue needs to be minimized. These two conditions led us to use aninhomogeneous, partially lossy dielectric lens. In a previous paper¹⁶,it was already shown that a multilayer dielectric lens can be used tomatch the impedance from air to tissue. The lens consists of five layersof different dielectric materials with dielectric constants varying inan exponential profile from air to the innermost layer, ϵ_(max). Due tothe increase of the dielectric constant, the focal spot size can bereduced by a factor of ϵ_(rmax) ^(−1/2) at the innermost layer, and theelectric field can be enhanced by a factor of ϵ_(max) ^(1/4). The choiceof the number of layers and their thickness was optimized to permitmaximal transmission through these layers. Here, a similar lens wasused, but with two major differences: 1) tissue with a givenconductivity was inserted into the innermost layer; and 2) the innermostlayer and the second to the last layer of the lens were made lossy forangles below 45°, which means, for azimuth angles between −45° and +45°,it has a finite conductivity (1 S/m or 2 S/m). The dielectric constantsof the materials were kept the same as in the previous lens design. Withthis design, the electric field distribution has been simulated for thecases where the lens does and does not include lossy layers.

FIG. 11 shows the field distribution of impulse for tissue conductivityof 0.5 S/m when the lens has no lossy layers. On the z-axis, theelectric field decreases steadily as the distance increases from thesurface. For paths of larger angles, there is an increase as thedistance increases. In the case where two layers were made lossy (1 S/mfor both layers), the field distribution is changed (FIG. 12A) due tothe strong attenuation of the lossy layer, the field distribution (forexample θ=0°, 30° becomes lower on the surface than for the case wherethere are no lossy layers in the lens. The field intensity on thesurface at larger angles becomes larger. But overall, the fields alongall radial paths show a decreasing trend as the distance increases.However, when the conductivity of the second last layer is increasedfrom 1 S/m to 2 S/m, the electric field intensity on the z-axis becomessmaller on the surface, but increases at a depth of 6 cm (FIG. 12B).Meanwhile, the radial distributions along other radial paths near thez-axis also show a slight increase in the deeper area, except on thesurface of the tissue at angles >60°. We note that such distribution isfor the tissue conductivity of 0.5 S/m, and for the tissue conductivityof 1 S/m, a similar field distribution was obtained. The fielddistribution can be schematically shown in FIG. 13A. In the case of anon-lossy lens, the field distribution can be schematically depicted bythe pie-shaped distribution in FIG. 13B. A comparison of FIGS. 13A and13B shows a localized focus may be formed in the deep region of thetissue. This change in distribution is made at the cost of the reductionof the absolute field intensity at the targeted region and two extra“hot spots” near the surface. It has been shown above that it ispossible to confine the electric field of subnanosecond pulses in ahomogeneous, dielectric hemisphere. The attenuation loss of the targetcan be offset by using a partially lossy lens so that a deep-seatedfocal region can be created. While the targeted hemisphere is simple instructure, a real tissue contains multiple tissue layers, making theformation of a focal spot more challenging. Three cases have beeninvestigated: 1) the subnanosecond pulses are directed to the brain bythe ellipsoidal reflector only; 2) a lens is placed on top of a humanhead; and 3) the lens is made partially lossy in order to attenuate thefield along the axis. The lens, with the same structure as in FIG. 3, isused with the antenna and placed on top of the human head, asillustrated in FIGS. 2A and 2B. The dielectric constant of each layerwas the same as in FIG. 3. Because the head is not exactly round inshape, the top of the head coincides with the lens axis. So the headappears to project into the lens structure. But at the lens-headinterface, the priority is given to the head, which overrides the lensin any overlapped region. For the lossy lens, which is similar to thelens used in Section 3.2, the innermost layer is assigned a conductivityof 5 S/m and the second-last layer is assigned a conductivity of 4 S/m.The values of the conductivities were selected based on theconsideration that the electric field on the axis increases as the wavepenetrates deeper and nears the geometric focus. However, that does notmean that the electric fields in other radial paths of the penetrationare lower than the fields near the geometric focus, as will be shownlater.

First examined is the case where the pulse is directly (without lens)delivered to the brain through the impulse radiating antenna. FIG. 14shows the pulse waveform at the geometric focus in the brain (6 cm deepfrom the top of the skull). Again, the y-component dominates the overallfield and the x and z components are negligible.

FIG. 15 shows the propagation of subnanosecond pulses in the head. Inparticular, FIG. 15 shows isoline plots of electric field distributionin the human brain without a dielectric lens at various times (1.8 ns to3.7 ns), where the top, middle, and bottom rows are view along theX-axis (side view), Y-axis (back view), and Z-axis (top view),respectively. The intensity of the electric fields is shown as thelinear isoline in the clamped range 0.1-0.3 V/m, meaning any values thatare greater than 0.3 are clamped to 0.3. At 1.8 ns, the prepulse alreadyreaches the brain. In the meantime, the prepulse in the face and backside of the head travels faster than the prepulse in the brain. At 3.1ns, the impulse appears in the brain along the axis of the antenna. Itgradually reduces its size and at 3.7 ns, seems at its smallest size. Asthe wave penetrates deeper, the intensity decreases. The isoline plotclearly indicates that there is no resonance in the brain except theimpulse. In the back view of the wave propagation (FIG. 15), at 3.1 ns,one can see that the impulse actually consists of two parts indicated bytwo bright spots. In the time domain, the corresponding impulse waveformis bipolar (FIG. 14). At 3.7 ns, the wave converges, which is consistentwith the side view. A top view of the wave propagation is also shown inFIG. 15 in the bottom row. The Z-axis views are taken at depths of 6, 7,8 and 9 cm from the top of the head. Because the wave intensity is shownas a clamped isoline, a number of probes were placed in the brain tomeasure the actual peak amplitudes as the pulse passes. The probes wereplaced in 1 cm steps along the radius centered at the second geometricfocal point, which is 6 cm deep from the top of the skin fat.

FIG. 16 shows the field distribution along various radii from the skin.The largest field is on the axis (0°) and the fields on the two sidesseem symmetrical about the axis. They all decrease due to the strongabsorption in the brain. Despite the fact that the field decreases asthe penetration depth increases, the field values converge at 7-8 cm, asign of geometric focusing in the space, which is also consistent withthe results shown in FIG. 15. The inhomogeneity of the brain does notseem to create any difference in the arrival time of the spherical wavesalong different radii, but the strong attenuation of brain tissueprevents the wave from reaching the highest amplitude near the focalpoint.

FIG. 17 shows the isoline plots of electric field distribution of thehuman brain under a dielectric lens at various times along the X-axis(side view), Y-axis (back view), and Z-axis (top view), shown in thetop, middle, and bottom rows, respectively. The top-view slice was takenalong the plane at 8 cm deep in the head. The left panel is for thenon-lossy lens and the isoline plot has a clamp to range of 0.1 to 0.5V/m. The right panel is for the lossy lens and the isoline plot has aclamp to range of 0.1 to 0.2 V/m.

For a non-lossy lens placed on top of the head, the resulting fielddistribution in isoline plots are shown in the left panels of FIG. 17.In this case, the isoline was clamped between 0.1-0.5 V/m. At 3.7 ns,the impulse becomes constricted in size and it reaches 8 cm deep at 3.9ns. After that, the impulse decreases in intensity and becomes almostinvisible in the clamped range. The addition of the lens creates a delaycompared with the case without a lens. The amplitudes of the field areshown in FIG. 18. The field distribution is similar to the case wherethere is no lens added, except the amplitude at the skin is slightlyhigher due to the higher transmission of the lens. The field valuesconverge to 0.5 V/m at the depth of 8 cm. Still, the highest field is onthe axis and the fields on both sides are smaller. The overall trend isdecreasing as the impulse penetrates.

For a lossy lens placed on top of the head, the resulting fielddistribution in isoline plots are shown in the right panels of FIG. 17.In this case, the field on the axis is attenuated by assigningconductivities to the 4th and 5th layer of the lens. In the side view ofthe wave propagation (FIG. 17, right panel, first row), the impulsefield, shows a “wishbone-like” distribution (at 3.7 ns). At 3.9 ns, theimpulse is confined at 8 cm from the top of the skull. In FIG. 19, thefield distribution is shown. On the axis (R vs 0°), the field becomesthe lowest among all the radial directions and is slightly increased ata 8 cm, which is the opposite of that in FIG. 16 and FIG. 18, where thelargest field is on the axis. The fields at ±45° become the largest. Thestrong attenuation in the lossy lens allows us to modify the fielddistribution in the brain. Despite the strong attenuation, the fieldintensity near the focal point (6-8 cm in depth) is 0.2 V/m, reduced bya factor of three from that in the non-lossy lens case (FIG. 18).

When sending subnanosecond pulses to a tissue, such as brain tissue,using an impulse antenna, a spherical wave with a propagation vectorperpendicular to a spherical target converges on the geometric center ofthe spherical wave. This offers a rather simple prediction of the fielddistribution in the target. The conductivity of the tissue howeverchanges the geometric-optics picture. It creates a decreasing electricfield with increasing penetration distance. For the generation of largefield intensities in the shallow region of the brain, a single antennais sufficient. As the antenna radiation is an inhomogeneous sphericalwave, the highest field is along the axis, which means the highest fielddistribution in the target also coincides with the axis. For deeperfocusing, the loss due to the conductivity of the dielectric can bealleviated by applying this method to tissue with high dielectricconstants, such as muscle tissue.

In a target which contains composite tissues, such as the brain in thisstudy, the multiple layers in the propagation path (skin and skull) donot pose a significant change to the waveforms of the converging waves,since, in FIG. 14, both prepulse and impulse can be clearly identified.In addition, the focal spot size in the brain is mainly determined bythe white matter, where the spherical wave converges. As white matterhas a dielectric constant of 37, the spatial width of such pulse is 1cm. The simulation shows that the spot size is on the order of 2 cm forinput pulse duration of 200 ps. It is therefore reasonable to predictthe focal spot size simply by estimating the pulse spatial width in thedielectric where the focal point is to be created.

The electric field distribution obtained in a hemispherical tissue(FIGS. 11 and 12B) and a brain model (FIGS. 18 and 19) demonstrates twoextreme cases of electric field distributions. One has the highest fieldon the z-axis while the fields on the side are small. This case createsa field distribution without a deep maximum. The other case is that highfields penetrate from the two sides about the z-axis, while leaving theaxial field small, but in the deep region, a local maximum can beformed. The first case can be used for delivering subnanosecond pulsesto the shallow regions along the axis and the second case can be usedfor delivering subnanosecond pulses to the shallow region on the sides.The targeted region can be selected by placing lossy materials in thecorresponding incident angles of the lens, which offers one way ofvarying the focal zone—by utilizing different lenses. On the other hand,the second case suggests that it may be possible to focus the pulse indeep regions and reduce the fields on the sides. This could be done byoptimizing the configuration of the lens. Such a lens could consist of anumber of lossy elements in the same layer of the lens with each layerhaving different values of conductivities. A typical multi-variableoptimization method, such as a generic algorithm³⁰, can be used to findthe optimum values of the lens segments.

Previous studies of the biological effects near 1 ns mostly were focusedon the electric field range, 16-250 kV/m²¹. The findings reported weremostly negative. Various aspects, including regulation of heart rate andblood pressure^(22, 23) chemically induced convulsions²⁴, and behavior,hematology, and brain histology²⁵, showed no response to the shortpulses. A behavioral study in primates²⁶, and yeast²⁷, also did not showany effects.

In these physiological studies, the electric fields were presumably low.When the electric field strengths were increased by 10-fold, up to 1.2MV/m, muscle stimulation became feasible, as shown in a later study²¹.Even single 1 ns pulses with electric field strength 1.2 MV/m were ableto stimulate frog gastrocnemius muscles. The strength duration curve(S-D curve) from 1 ms to 1 ns showed a linear trend, which suggests thefeasibility of stimulating excitable tissue into the subnanosecond(picoseconds) range. A recent study²⁸ using longer pulses (12 ns) alsoshowed that single electric pulses were able to elicit action potentialswhen the pulses were applied to rat skin nociceptors. The criticalelectric field strength of 403 V/cm was found to be dependent on thefrequency and burst duration. When the electric pulses were applied in arepetitive, burst manner, the electric field strength threshold ofstimulation was reduced to 16 V/cm for a 4 kHz, 25 ms burst.

Using the same linear scaling of Rogers et al.²¹, one can extrapolatethe critical electric field for stimulation with 200 ps to be 6 MV/m. Anestimate within an order of magnitude can be made to be 1 MV/m -10MV/m.On the other hand, as Jiang and Cooper showed²⁸, the critical electricfields may decrease by a factor of 20 if the pulses are delivered at ahigh rep-rate (4 kHz). Therefore, an electric field of 50 kV/m and 500kV/m may be sufficient for stimulation. These values are within thecapability of antenna delivery. As shown in FIG. 16 and FIG. 18, for avoltage of 1 V fed to the antenna, an electric field is in the range of1.5 V/m at a depth of 2 cm, which is the motor cortex region in thebrain. This means an input voltage from 33.3 kV to 333 kV may be enoughfor an effective stimulation. For deep stimulation, FIG. 11, FIG. 13 andFIG. 14 indicate there is a locally focused region at a depth of 7-8 cm.For a 1V pulse input, the field intensity was found to be 0.2-0.5 V/mfor the three cases, which suggests that pulses of 250 kV-2.5 MV need tobe fed into the antenna in order to meet the estimated minimumstimulation threshold of 50 kV/m. Such a high-amplitude pulse generatorbecomes technically challenging (if not impossible) even though a 1 MVpulse generator has been built³¹.

In summary, the electric field distribution shows a steadily decreasingtrend and is primarily caused by the tissue conductivity. But increasingthe tissue dielectric constant, meaning applying the method to adifferent tissue, can reverse the trend and allows for a deep focusing(6 cm in depth for example). A second approach that could possiblycreate deep focusing is to use a dielectric lens in conjunction with theantenna. The lens can modify the electric field distribution in thetissue. In one case, the lens is loss-free and the electric fields alongthe axis are the highest. In the other case, the lens is assigned withlossy material to attenuate the axial incident electric fields, butallowing the incident fields from the sides to pass. This case resultsin the opposite distribution as the first case: the largest fields onthe side, and the small fields along the axis. A local maximum near thegeometric focus exists in this case. When the target is replaced by ahuman brain, the field along the axis is still strongest if the humanhead is directly exposed to the antenna. Despite a slight asymmetry ofthe human head, the spherical wave still converges near the geometricfocus. However, the overall intensity decreases from the surface as thewave penetrates, due to a strong resistive loss of the tissue. Adding adielectric lens allows one to increase the field intensity near thegeometric focus, but the strongest intensity is still found along theaxis and decreases as the wave penetrates deeper. When the lens is madelossy, the field on the axis can be adjusted to the lowest in the brain,while leaving the field distribution near the ±45° largest. This studysuggests the possibility of designing a dielectric lens withheterogeneous distribution of lossy material to create a desirableelectric field distribution in the brain, perhaps even allowing fordeep-focusing. Future work will be embedding lossy elements to the lenslayers and steering the antennas to move the focal point in the brain.

While various embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample only, and not limitation. Numerous changes to the disclosedembodiments can be made in accordance with the disclosure herein withoutdeparting from the spirit or scope of the invention. Thus, the breadthand scope of the present invention should not be limited by any of theabove described embodiments. Rather, the scope of the invention shouldbe defined in accordance with the following claims and theirequivalents.

Although the invention has been illustrated and described with respectto one or more implementations, equivalent alterations and modificationswill occur to others skilled in the art upon the reading andunderstanding of this specification and the annexed drawings. Inaddition, while a particular feature of the invention may have beendisclosed with respect to only one of several implementations, suchfeature may be combined with one or more other features of the otherimplementations as may be desired and advantageous for any given orparticular application.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. Furthermore, to the extent that the terms “including”,“includes”, “having”, “has”, “with”, or variants thereof are used ineither the detailed description and/or the claims, such terms areintended to be inclusive in a manner similar to the term “comprising.”

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this invention belongs. It will befurther understood that terms, such as those defined in commonly useddictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of the relevant art andwill not be interpreted in an idealized or overly formal sense unlessexpressly so defined herein.

REFERENCES

The following references describing information for understanding thevarious embodiments of the invention. Each of these references is hereinincorporated by reference in their entirety.

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What is claimed is:
 1. A system configured to deliver electric pulses toa targeted region within a biological tissue, comprising: an antennaassembly configured to generate and direct electromagnetic radiation; alens configured to be positioned between a surface of the biologicaltissue and the antenna assembly, the lens having a plurality of lossyportions; wherein the lens is configured to be adjustable to create apatient-specific desired electric field distribution by selectivepositioning of the plurality of lossy portions within the lens.
 2. Thesystem of claim 1, wherein the lens comprises a plurality of layers,each layer having a different value of conductivity.
 3. The system ofclaim 2, wherein the plurality of lossy portions is positioned within asingle layer of the plurality of layers.
 4. The system of claim 2,wherein the plurality of lossy portions is positioned within multiplelayers of the plurality of layers.
 5. The system of claim 1, wherein theplurality of lossy portions is positioned at various incident angles ofthe lens.
 6. The system of claim 1, wherein each of the plurality oflossy portions is configured to attenuate a portion of theelectromagnetic radiation traveling in at least one direction.
 7. Thesystem of claim 1, wherein the targeted region is a deep tissue regionof the biological tissue and wherein the system is configured toincrease a transmission of the electromagnetic radiation to the deeptissue region of the biological tissue.
 8. The system of claim 7,wherein a focal spot in the deep tissue region is generated byattenuating the electric fields on or near a z-axis.
 9. The system ofclaim 1, wherein the deep tissue region comprises a region of 6 to 8 cmbelow a surface of the biological tissue.
 10. The system of claim 1,wherein the system configuration simultaneously targets both deeper andshallower regions of the biological tissue.
 11. The system of claim 1,wherein the electromagnetic radiation from the antenna assembly isconfigured to enter the targeted tissue at wide angles.
 12. The systemof claim 11, wherein the wide angles are equal or greater than 45degrees.
 13. The system of claim 1, wherein the antenna assemblycomprises a wide-band antenna configured to provide wide and narrowangles.
 14. The system of claim 1, further comprising a pulse generatorcoupled to the antenna assembly and configured for generating a seriesof pulses.
 15. The system of claim 14, wherein the pulse generator isconfigured for generating a pulse having a pulse duration of less thanor equal to 1 ns.
 16. The system of claim 14, wherein the pulsegenerator is configured for generating a pulse having a magnitudebetween about 1 kV and 1 MV and a repetition rate between about 1 Hz and1 MHz.
 17. The system of claim 1, wherein at least one of a rise time ora fall time of the electric pulses is from 1 ps to 1 ns.
 18. The systemof claim 1, wherein the lens comprises a plurality of layers and atleast one of a radius, thickness, or dielectric constant of each layerof the plurality of layers varies.
 19. The system of claim 1, whereinthe antenna assembly comprises an antenna configured for receiving theelectric pulses and generating the electromagnetic radiation and areflector configured for directing the electromagnetic radiation to thelens.
 20. The system of claim 1, wherein the lens is shaped andconfigured for positioning adjacent to the biological tissue and adielectric constant of a surface of the lens that is adjacent to thebiological tissue substantially matches a dielectric constant of thebiological tissue.